The design of the onboard powertrain energy storage system (ESS) plays a vital role in laying out performance specifications for an electric vehicle (EV). One solution to address the relatively low power density problem of battery being sole storage source is to hybridize ESS with higher power density sources such as supercapacitor (SC) via a buck-boost converter to electrically regulate each power flow bi-directionally. This paper proposes a simple but innovative method with the aim of guiding EV designers of logically choosing the optimal size and ratio of each power source to form an active hybrid energy storage system (HESS), when considering a set of constraints in practice. This method is then tested comprehensively by a typical design case, where we combine a high voltage LiFPO4-based battery pack with an auxiliary supercapacitor pack while interfacing them with an intermediate buck-boost converter, before connecting the whole system to the load. Global optimal solutions are proved to be found using this method to achieve meeting the pre-set performance requirements at minimum system size and cost.